Heterodyne receiver



Patented Nov. 3, 1936 PATENT OFFICE HETERODYNE RECEIVER Frederick H. Drake, Boonton, N; J., assigner to Radio Corporation of America, a corporation of Delaware Application May 3, 1935, Serial No. 19,575

Claims.

My present invention relates to heterodyne receiving systems, and more particularly to heterodyne receivers o-f the audible beat type.

There are various factors which must be con- 5 sidered inthe design of heterodyne receiving systems of the audible beat type. As is Well known, such a receiving system usually employs in addition to amplifiers operating at super-audible frequency, and an audio amplifying system, a detector which has impressed thereupon locally produced oscillations. The frequency of the latter oscillations differ from the desired signal frequency by a beat note in the audible frequency range. Inthe utilization of such a receiving system it is not only desiredthat the beat note be produced with maximum loudness and with minimum use of tubes, but, also, with minimumA of regenerative r-eaction in the heterodyning circuits in order to prevent the production of spuriyous Whistl-es. Another operating defect which commonly occurs in receiving systems of this type may be termed locking, or sucking-in, of the oscillator frequency by the signal frequency. Such sucking-in action .arises because the signal frequency draws the oscillator frequency into synchronization with itself. When it is rememberedthat the two frequencies are very close to each other, .and only differ by a frequency of the order of a tone such as 100.0 cycles, it is readily appreciated why such synchronization action is undesired. This locking action manifests itself chiefly by a ragged note above a certain audio frequency limit. Below this limit the note disappears. The more the sucking-in the higher the minimumaudio beat frequency which can be heard. If this action is too bad, it may be impossible to hear a beat as low as 1000 cycles. Accordingly, it may be stated that it is one of the primary objects of the present invention to provide Aa heterodyne receiving system of the audible beat type which not only effectively prevents the sucking-in action of the impressed signal on the local oscillator, but also results in the production of an audible beat with high gain and with a minimum of apparatus, the method utilized inthe present invention briefly involving the impression of the locally produced oscillations upon the suppressor grid of the receiver detector through a buffer amplifier. 50 Ano-ther important object of the invention is to provide a heterodyne receiving system which includes, inaddition to a detector, a super-audible frequency amplifier, the system further including a local oscillator, and the detector being of the pentode type, the locally produced oscillations be- (Cl. Z-8) ing impressed upon the suppressor grid through an electron discharge ampliiier functioning `as a buffer stage, While the signals are impressed upon the signal grid of the detector, the receiving system further including an automatic volume con- 5 trol tube whose electrodes are disposed in the same tube envelope as the electrodes of the buffer amplifier.

Another object ofthe present invention may be stated to involve a coupling ariangement between 10 the suppressor grid of a pentode tube detector and a local oscillator, the coupling being designed to permit the balancing out of the suppressor grid to signal control grid capacity existing in the detector tube.

Still other objects of the invention are to improve generally the efciency of heterodyne receivers, and` more especially to provide an improved heterodyne receivingsystem of the audible beat type which is not only reliable and efficient in operation, but readily and economically constructed.

The novel features which I believe to'be characteristic of my invention are set forth in particularity in the appended claims, the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated'diagrammatically several circuit organizations whereby my invention may be carried into effect.

Fig. 1 diagrammatically shows a heterodyne receiving system embodying the present invention,

Fig. 2 shows the detector and local oscillator circuits of a heterodyne receiving system embodying a modification of the invention.

Referring now to the accompanying drawing, wherein like reference characters in the two figures designate similar circuit elements, it will be noted that there is shown in Fig. 1 a heterodyne receiving system whose networks, in general, follow a conventional pattern. The numeral l designates a source of signals, and this source may be the usual grounded antenna circuit, a loop antenna, an airplane antenna, or any other device for collecting radiated C. W. signals. -The detector tube 2 which is a pentode tube of the 77 type, has its signal input electrodes coupled to the source of signals I through an amplifier network. 'I'he amplifier network operates at superaudible frequency, and may comprise one, or more stages of amplification. The input circuits of the super-audible frequency amplifiers, which are designated as R. F. amplifiers and the detector have their input circuits tuned. The various tuning condensers of the receiving system are to be uni-controlled; such uni-control devices are well known to those skilled in the art.

'I'he detector tube 2 includes in its anode circuit a load resistor 3, and the beat voltage developed across resistor 3 is impressed upon an audio network (not shown). The latter may comprise one, or more, stages of amplification followed by a reproducer of any desired type. The desired signal energy is impressed between the signal control grid and cathode of the tube 2, and the cathode circuit of the tube includes the usual grid bias resistor-shunt condenser network. The screen grid electrode of the tube is connected to a desired point of positive potential on the B voltage supply source 4, the screen grid circuit being by-passed to ground through a radio frequency by-pass condenser.

The local oscillator of the receiving system comprises an electron discharge tube 5 which is of the type. It will be recognized that the oscillator involves a purely conventional circuit arrangement, and for this reason it is not believed necessary to describe the elements thereof. It is believed sufficient to point out that the tuning condenser thereof tunes the local oscillator network to a frequency differing from the signal frequency by a frequency of the beat note which is desired. The local oscillator network produces local oscillations by virtue of the regenerative coupling M between the cathode circuit of tube 5 and the tuned grid circuit thereof. The locally produced oscillations are impressed upon the suppressor grid 6 of the pentode tube 2. However, they are not directly impressed upon the suppressor grid, but are transmitted thereto through the agency of a buffer amplifier. This is done in order to prevent sucking-in action of the impressed signal frequency on the oscillator frequency.

The electrodes of the buffer tube are housed within the envelope 'I of a tube of the 79 type. Such a tube is well known to those skilled in the art at the present time, and comprises a pair of independent triodes disposed within a common tube envelope. The cathodes of the two triode systems are connected together, as by means of lead 8, and are at a common potential. The grid II of the buffer triode of tube 'I is connected to the cathode II thereof through a path which includes coil 9, the latter being magnetically coupled to the tuned grid circuit of the local oscillator, and lead Ill. The output electrode I2 of the buffer triode is connected to ground through a resistor I3. The suppressor grid 6 of detector tube 2 is connected to the anode side of resistor I3 through a lead I4.

The remaining triode system of tube I functions as an automatic amplification control tube, and the grid thereof is connected to the signal input grid of detector tube 2 through a condenser I5 which passes signals to the first named grid. The plate, or output electrode, of the second triode system is connected to ground through a resistor I6, the latter being by-passed for radio frequencies by condenser I'I, and a gain control connection, designated AVC, is connected between the plate side of resistor I6 and the signal grid circuits of the super-audible amplifiers whose amplification is to be regulated in response to Variations in amplitude of incoming signals. The sensitivity of these gain-controlled amplifiers is a maximum for weak signal reception, and, as the received signal amplitude increases, the function of the automatic gain control triode and its associated circuit is to reduce the sensitivity, or gain, of each of the super-audible amplifiers.

By way of illustration, it is shown that the common cathode lead of tube 'I is connected to a point which is at volts with respect to ground, whereas the signal input grid of the automatic gain control triode section of tube 'I is connected to a point at volts with respect to ground through resistor I8. It will be understood that the usual common voltage supply bleeder resistor may be used for the circuits of tube 'I.

It will now be observed that the locally produced oscillations are impressed upon the input electrodes of the buffer triode section of tube I, and that the output of the buffer section is impressed upon the suppressor grid of the detector tube 2. The resistor I3 is used as the impedance in the plate circuit of the buffer stage to provide substantially equal amplification at the various frequency ranges of the receiver. If this resistance I3 is given a value of between '750 to 1000 ohms the effect of the varying capacity reactance I3', shown in dotted lines, in parallel with it will be relatively slight, and approximately as much voltage will be available in the plate as is applied to the grid II. The AVC triode section of tube 'I is capable of functioning during the operation of the buffer section; the electrodes of the two sections are disposed within a common tube envelope and this is a very desirable form of construction since it enables the designer to dispense with separate tubes for these two functions.

An additional advantage of the arrangement shown in Fig. 1 resides in the fact that there is provided a method of giving negative bias to the suppressor grid 6 of the detector tube 2 conveniently, the bias increasing the suppressor modulation of the detector. In other words the buffer stage, and its impedance in the plate circuit thereof, not only functions to prevent the sucking-in action of the impressed signal on the oscillator when the heterodyning voltage is introduced on the detector, but it also functions to provi-de the suppressor grid bias which increases the eiiiciency of suppressor modulation action in the detector tube. The arrangement disclosed results in much less sucking-in action over all frequency ranges when the stray capacity couplings across the structure of the tube 'I do not produce too much voltage -on the control grid of the AVC section of tube 'I, and hence the detector. The buffer section of tube 'I may employ a screen grid between plate I2 and grid II to improve the action of the invention.

With efcient shielding in the tube 'I, or rather shielding between the metal cap electrode (this electrode being the signal input grid of the AVC triode section) and the other tube elements, this arrangement functions in a satisfactory manner. Although for a multi-range frequency receiver the value of resistance I3 is chosen so low that practically no amplification is obtained, the sucking-in action is almost entirely eliminated. Of course, those skilled in the art are fully acquainted with the manner of providing the switching arrangements to have the receiving system shown functioning in more than one frequency range, and it is to be clearly understood that the present invention is readily applicable to such type of multi-range receiver.

In Fig. 2 there is shown a novel alternative method of reducing the sucking-in action. In this method the local oscillator network, which includes tube 5, is coupled to the suppressor grid of the detector tube 2. The purpose of this arrangement is to permit balancing out the small suppressor grid to signal control gri-d capacity which exists in the electrode structure of tube 2. This inter-electrode vcapacity is designated in dotted lines, and is represented by the symbol C1. The capacity C1 may be balanced out provided the stray capacity between the local oscillator grid andthe signal input grid of tube 2 is properly proportioned. This stray capacity is shown in dotted lines, an-d is designated by the symbol C2. A balance of these capacity effects can be secured if the ratio of coupling between coils Lz and La to the inductance of coil L2 is made equal to the ratio of the capacity C1 to the capacity of C2.

It will be noted that the oscillator network in Fig. 2 provides local oscillations by means of the regenerative coupling between the coils L2 and L1. The coil L3 may be a portion of the coil section L1, and coil -L3 is magnetically coupled to coil L2. If the magnitude of the inter-electrode capacity C1 is as low as 9.15 micromicrofarad the balancing arrangement shown in Fig. 2 can be dispensed with, since by proper choice of biases enough heterodyne action can be developed through the suppressor, in the absence of any such balance, without producing more than about 0 .3 v-olt (R. F.) on the detector signal control grid.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. In combination with a detector tube of the pentode type, the latter including a suppressor grid disposed between the output electrode thereof and a positive screen electrode, means for impressing desired signals upon the input electrodes of the detector tube, a separate vacuum tube oscillator for producing local oscillations differing in frequency from the signal frequency by an audible frequency, a path c-oupled between said local oscillation means and the suppressor grid of said detector tube for impressing the locally produced oscillations upon said suppressor gri-d, said path including an electron discharge tube having its input electrodes coupled to said oscillator, and its output circuit conductively connected to said suppressor grid.

2. In combination with a detector tube of the pentode type, the latter including a suppressor grid disposed between the output electrodes thereof and a positive screen electrode, means for impressing desired signals upon the input electrodes of the detector tube, a separate vacuum tube oscillator for producing local oscillations diering in frequency from the signal frequency by an audible frequency, a path coupled between said local oscillator and the suppressor grid of said detector tube for impressing the locally produced oscillations upon said suppressor grid, said path including an electron discharge device having an impedance in its space current path, the input electrodes of said device being coupled to said oscillator, and a direct current connection connecting the suppressor grid of the detector tube to one side of sai-d impedance.

3. In combination with a detector tube including at least a cathode, signal grid and a suppressor grid disposed between the output electrodes thereof and a positive screen electrode, means for impressing desired signals upon the signal grid of the detector tube, means for maintaining said signal grid at a substantially constant negative bias voltage with reference to said cathode, means for producing local oscillations differing in frequency from the signal frequency by an audible frequency, a path coupled between said local oscillation means and the suppressor grid of said detector tube for impressing the locally produced oscillations upon said suppressor grid, said signal impressing means including at least one stage of signal amplification, means for automatically regulating the degree of amplication of the latter in accordance with the strength of the desired signals, and said path and last named means including electron discharge devices whose electrodes are disposed within a common tube envelope.

4. In combination with a signal detector of the pentode tube type, a local oscillator including a tube provided with regeneratively coupled input and output circuits, said pentode tube including a spurious capacity coupling between the signal input grid and suppressor grid thereof, a coil connected to said .suppressor grid, and means coupled to said detector and local oscillator for balancing out the effect of said spuriousl coupling, said balancing means comprising properly proportioned stray capacity coupling between the signal control grid of the detector tube and local oscillator grid, and magnetic coupling between the oscillator input circuit and said coil connected to the detector suppressor grid.

5. In combination with a detector tube of the pentode type including a suppressor grid disposed between the' output electrodes thereof, and a positive screen electrode, means for impressing desired signals on the input electrodes of said tube, a separate vacuum tube oscillator for producing local oscillations differing in frequency from the signal frequency by an audible frequency, a path coupled between said local oscillator and said suppressor grid for impressing the locally produced oscillations upon said suppressor grid, said path comprising a. vacuum tube having a cathode, control grid and plate, an input circuit connecting said control grid and cathode, means coupling said input circuit to said oscillator tube source, a resistor connected to said plate and in the space current path of said tube, and a direct current connection between the low potential end of said resistor and said suppressor grid.

FREDERICK H. BRAKE. 

